The present invention relates to a semiconductor device composed of a group III-V nitride semiconductor and, more particularly, to a semiconductor device having an ohmic electrode formed on a group III-V nitride semiconductor layer and to a method for fabricating the same.
Studies have been made on the application of a group III-V nitride semiconductor, i.e., a mixed crystal compound semiconductor represented by the general formula AlxInyGa1-x-y (where x and y satisfy 0≦x≦1, 0≦y≦1, and 0≦x+y≦1) not only to an optical device using a wide band gap and a direct transition band structure, which physically characterize the mixed crystal compound semiconductor, but also to an electronic device using a high breakdown electric field and a high saturation electron velocity, which also characterize the mixed crystal compound semiconductor. In particular, a hetero-junction field effect transistor (hereinafter referred to as HFET) using a 2-dimensional electron gas (hereinafter referred to as 2DEG) appearing at the interface between an AlxGa1-xN layer (where 0<x≦1 is satisfied) and a GaN layer, each epitaxially grown on a semi-insulating substrate, as well as a hetero-junction bipolar transistor (hereinafter referred to as HBT) have been developed progressively as high-output RF devices.
In the HFET, an electron density is over 1013 cm−2 due to electrons supplied from a carrier supply layer made of, e.g., AlGaN and serving also as an n-type barrier layer and to charges resulting from a polarization effect (spontaneous polarization, i.e., piezo polarization). The electron density is higher than that in a HFET made of an AlGaAs-based compound semiconductor by about one order of magnitude.
In addition, a group III-V nitride semiconductor such as gallium nitride (GaN) features a high breakdown voltage characteristic due to its relatively large band gap energy of 3.4 eV. Because such electric characteristics as a high breakdown voltage and a high current density can be expected, studies have been made on the application of electronic devices using group III-V nitride semiconductors typically represented by the HFET and HBT as ultra-high-speed and large-output devices in smaller sizes.
Although the electronic devices using group III-V nitride semiconductors are thus promising as ultra-high-speed devices and large-output devices, various improvements and modification should be made to realize ultra-high-speed and large-output devices by using such nitride semiconductors.
As a first problem to be solved to realize an ultra-high-speed and large-output device using a group III-V nitride semiconductor, a reduction in contact resistance can be listed.
A description will be given herein below to a conventional GaN semiconductor device (HFET) using a group III-V nitride semiconductor with reference to FIG. 16 (see, e.g., Japanese Laid-Open Patent Publication No. 2003-59946).
As shown in FIG. 16, a GaN semiconductor device A has a first semiconductor layer 3 made of undoped GaN and formed on a substrate 1 with a buffer layer 2 interposed therebetween and a second semiconductor layer 4 made of undoped AlGaN which is larger in band gap energy than the first semiconductor layer 3 and formed on the first semiconductor layer 3. Consequently, a 2DEG layer 6 is formed in the region of the first semiconductor layer 3 located in the vicinity of the interface with the second semiconductor layer 4.
A Schottky gate electrode G is formed on the second semiconductor layer 4. The upper portions of the regions of the second semiconductor layer 4 located on both sides of the gate electrode G are partly removed such that two contact areas 4A are formed. A source electrode S and a drain electrode D, each of which is an ohmic electrode, are formed on the respective contact areas 4A. The exposed surface of the second semiconductor layer 4 is covered with an insulating film 7.
Thus, the conventional GaN semiconductor device A forms the contact areas 4A as regions to be formed with the ohmic electrodes by partly removing the upper portions of the second semiconductor layer 4 for generating the 2DEG layer 6 in the first semiconductor layer 3 and thereby reduces the distance between the 2DEG layer 6 and each of the ohmic electrodes S and D such that the contact resistance between the first semiconductor layer 3 and each of the ohmic electrodes S and D is reduced.
However, the conventional GaN semiconductor device mentioned above has the problems of a damage to the second semiconductor layer 4 caused by etching during the formation of the contact areas 4A and a reduction in the concentration of an electron gas in the 2DEG layer 6 caused by the etching damage to the second semiconductor layer 4.
Although the surface of the second semiconductor layer 4, especially the surface portions thereof located on both sides of the gate electrode G are covered with the insulating film 7, a leakage current (gate leakage) easily flows between the gate electrode G and the second semiconductor layer 4 under the influence of a surface level resulting from the exposure of the end faces. As a result, the electric characteristics (operating characteristics) of the device deteriorate even though the contact resistance is reduced.
There is also the problem that it is difficult to determine the end point of etching for forming the contact areas 4A having a depressed configuration.